When a military aircraft flying in unknown or hostile air space is discovered by a distant radar apparatus it is often beneficial for the crew of the aircraft to not only be made aware of the occurrence of this radar discovery but to also be appraised of as much information regarding the discovering radar as is possible. Two significant portions of this discovering radar information are the physical location of and the operating frequency of the distant radar apparatus. In addition to the fundamental act of receiving such information concerning the discovering radar apparatus it is desirable that this information become available to the aircraft crew as quickly as is possible and that the information be obtained from as little as one pulse of energy received from a threat signal source. The obtaining of this and other information such as pulse duration and signal strength data in a passive non-signal radiating manner from a distant threat signal is the role of the electronic warfare radio receiver.
The radio receiver arrangement we have identified by the name of a xe2x80x9cmonobit receiverxe2x80x9d offers an attractive basis for fabricating such an electronic warfare receiver and for solving several problems arising in the electronic warfare and other military electronics fields of endeavor. One group of such problems is locating the source of a distant radio frequency emission from a single pulse of received radio frequency energy emission i.e., providing a radio frequency direction finding capability that is usable in the present day passive monopulse electronic signal environment. Although radio receivers technically capable of performing in this direction finding and frequency identification environment have existed for some time the cost, technical complexity and physical size of each such existing receivers and related problems such as relatively short intervals of mean time between receiver failure events have limited the practicality of direction finding apparatus using existing electronic warfare receivers. This limitation is especially notable with respect to locating such apparatus within the confines of and within the weight limitations of a host aircraft such as a tactical or fighter aircraft.
It has been clear to persons working in the monopulse systems technical field that a passive instantaneous direction finding apparatus built around a multiple element directive antenna having elements coupled through a phase responsive network such as a Butler matrix to a plurality of individual radio receivers would be within the realm of technical possibility except for the penalty of cost, technical complexity and physical size associated with each of the individual radio receivers needed to embody such a system. Indeed persons working in this field have proposed such direction finding and frequency identification systems in considerable detail. One such system is for example disclosed in the 1996 U.S. Pat. No. 5,568,154 of Yakov Cohen of Haifa, Israel. The Cohen ""154 patent indeed involves a frequency and direction finding system inclusive of a multiple element directional antenna, a Butler matrix and radio receivers assembled into a combination providing first blush similarity to the system of the present invention.
A more detailed consideration of the Cohen direction finding and frequency identification system reveals however the use of several radio receivers of one of the types identified as xe2x80x9cchannelized receivers, Bragg cell receivers, compressive receivers (and) digital FFT receiversxe2x80x9d, see column 1, line 19 of the Cohen patent. Five of a selected one of such receiver types are included at 122-130 of the Cohen patent""s exemplary FIG. 1 direction finding system drawing. When the cost, technical complexity and physical size associated with each of these previous receiver types is considered, the limited utility of the resulting Cohen FIG. 1 system, particularly in a small aircraft, begins to emerge however. The inventors of the present invention have used receivers of these previous types in experimental laboratory work and in fact one of the present inventors has authored a published text book in which both technical characteristics and physical embodiment photographs of individual receivers of this type appear. See the Text xe2x80x9cMicrowave Receivers With Electronic Warfare Applicationsxe2x80x9d authored by James Bao-Yen Tsui, published by John Wiley and Sons, copyright 1986. Photographs of circa mid 1980""s versions of receivers of these channelized receivers, Bragg cell receivers, compressive receivers and digital FFT receiver types appear on pages 229, 330, 279 and 183 respectively in the Tsui text. From these photographs the physicalsize portion of the difficulties attending a system according to the Cohen patent, using five or more of such receivers, becomes apparent. In the interest of simplifying and shortening the present document nevertheless the contents of both the Cohen patent and the Tsui text are hereby incorporated by reference herein. At the very least these documents provide enlightening background and signal characteristics information. Another text providing helpful background information with respect to the present invention is the text xe2x80x9cMicrowave Passive Direction Findingxe2x80x9d authored by Stephen E. Lipsky, also published by John Wiley and Sons, and copyright 1987. The Lipsky text is also hereby incorporated by reference herein.
A significant part of the difficulty with the previous digital FFT receivers heretofore potentially used in monopulse frequency and direction-finding applications relates to the algorithm used to embody the Fourier transformation operation in the receiver. Most Fourier transformation realizations necessarily include an extensive use of numeric multiplication in computing values related to the kernel function portion,       ⅇ                            -          j2                ⁢                  xe2x80x83                ⁢        π        ⁢                  xe2x80x83                ⁢        kn            N        ,
i.e., the exponential of xe2x80x9cexe2x80x9d the base of the natural logarithm, within the Fourier transformation algorithm. Both the number of and the size of each individual of these multiplications contributes to the complexity of rigorously implementing the Fourier transformation in either hardware or software form and especially to the difficulty of implementing this operation in real time. In an effort to reduce this complexity one of the present invention inventors, James B.Y. Tsui and a number of colleagues, have shown that Fourier transformation Kernel functions of unit magnitude or substantially unit magnitude may be used to successfully approximate a true Kernel function value and enable the realization of a Fourier transformation using only multiplication by unity or in essence no multiplication in the Fourier transformation computation algorithm. Kernel function realization in this manner is disclosed in a first U.S. Patent of Tsui et al., a patent numbered U.S. Pat. No. 5,917,737, wherein Kernel function values are located on a circle of unit radius at angular locations of xcfx80/4, 3xcfx80/4, 5xcfx80/4 and 7xcfx80/4 radians.
A later patent document involving inventor Tsui and colleagues wherein the Kernel function values are moved on the circle of unit radius to locations of 0, xcfx802, xcfx80, and 3xcfx80/2 radians is identified as U.S. Pat. No. 5,963,164. In a yet later patent document, the U.S. patent application identified with Ser. No. 09/944,616 and filed on Sep. 4, 2001, inventor Tsui and a colleague have demonstrated advantages available when Kernel function values located at each of the xcfx80/4, 3xcfx80/4, 5xcfx80/4 and 7xcfx80/4 radian locations are added to the Kernel function values at 0, xcfx80/2, xcfx80, and 3xcfx80/2 radians with the added four values being slightly increased in magnitude from unit circle values and in fact having a magnitude of (2)1/2 or 1.414.
The incentive for improving the Kernel function approximations over that of the earlier U.S. Pat. No. 5,917,737 patent is ease of realizing the approximation in the transition of the U.S. Pat. No. 5,917,737 patent to that of the U.S. Pat. No. 5,963,164 patent and a desire for improved receiver signal amplitude tolerance or dynamic range response enhancement in the serial number instance. The FIG. 4 drawing herein shows the eight unit value-related Kernel function approximation locations disclosed in the Ser. No. 09/944,616, Sep. 4, 2001 application in graphic form and also demonstrates Kernel function locations usable in the present invention. These same eight Kernel function locations are also used in the invention of U.S. patent application of Ser. No. 10/008,476, applicants"" attorneys docket number AFD 481, filed in Dec. 2001.
Notwithstanding the previous attribute of having a somewhat limited two tone dynamic range characteristic the monobit receiver using limited Kernel function values is nevertheless believed an attractive arrangement for use in a passive microwave frequency direction finding system. Additional improvements and performance enhancements currently under investigation for this receiver suggest the possibility of even greater attraction to the monobit receiver configuration for direction finding use. The relatively low cost, small physical size and simplicity of any version of this monobit receiver are especially seen as welcome additions to the currently available selection between for example the channelized receivers, Bragg cell receivers, compressive receivers (and) digital FFT receivers identified in the Cohen patent document. The possibility that such a monobit receiver can be realized on a single integrated circuit chip especially makes a direction finder of the present invention type a realistic possibility and moreover greatly enhances the prospect of this apparatus being sufficiently small and light in weight as to enable its use in even a small military aircraft. Such a direction finder is the area of interest in the present invention. The direction finder of the present invention can of course also be used in other settings including use in connection with an unattended ground sensor or an unmanned air vehicle.
The present invention provides a simplified, small size, passive, instantaneous- operation, microwave direction finding and microwave signal frequency identification system.
It is therefore an object of the present invention to provide a simplified, small size, passive, instantaneous-operation, microwave direction finding and microwave signal frequency identification system that is based on a simplified unit value related approximation of the Fourier transformation Kernel function       ⅇ                            -          j2                ⁢                  xe2x80x83                ⁢        π        ⁢                  xe2x80x83                ⁢        kn            N        .
It is another object of the invention to provide a microwave direction finding and microwave signal frequency identification system that is compatible with use in a tactical military aircraft.
It is another object of the invention to provide a microwave direction finding and microwave signal frequency identification system that may be of selected accuracy and complexity.
It is another object of the invention to provide a microwave direction finding and signal frequency identification system in which the number of Fourier transformation receivers may be selected.
It is another object of the invention to provide a microwave direction finding and signal frequency identification system in which the number of receiving antenna elements and the system resolution capability may be selected.
These and other objects of the invention will become apparent as the description of the representative embodiments proceeds.
It is another object of the invention to provide a microwave direction finding and signal frequency identification system that may be used in both military and civilian endeavors.
These and other objects of the invention are achieved by the passive method of identifying both operating frequency and relative angular location of a distant source of microwave radio frequency radiant energy with respect to a receiving location, said method comprising the steps of:
receiving, in multiple elements of a circular disposed omni directional microwave antenna located in said receiving location, multiple antenna element samples of energy radiated from said distant source of microwave radio frequency radiant energy;
coupling electrical signals, generated by said received radiated energy in each of said circular disposed omni directional microwave antenna elements, through a mode forming electrical matrix to phase segregated multiple output ports of said mode forming electrical matrix;
communicating each of said mode forming electrical matrix phase segregated multiple output port electrical signals to a separate monobit electronic warfare radio receiver of substantially unit value Fourier transformation Kernel function   ⅇ                    -        j2            ⁢              xe2x80x83            ⁢      π      ⁢              xe2x80x83            ⁢      kn        N  
realization and signal phase angle preserving characterization;
determining, from Fourier transformation of each said communicated phase segregated multiple output port electrical signal in one of said monobit electronic warfare radio receivers, a predominant signal frequency component of said energy radiated from said distant source of microwave radio frequency radiant energy;
ascertaining, from phase decoding of Fourier transformations of multiple of said communicated, phase segregated, multiple output port electrical signals from said electronic warfare radio receivers, an angle of arrival vector, with respect to said receiver location, for said energy radiated from said distant source of microwave radio frequency radiant energy.